It concerns more particularly systems in which a spatial modulator is used in connection with a laser source to produce a column of individually controllable spots as described in U.S. Pat. No. 4,746,942. This column of spots is moved transversally to the motion of the media to produce successive adjacent bands as shown in FIG. 3 of said patent. In order to avoid any noticeable discontinuity between successive bands it is not only necessary that the relative displacement of the bands and the media corresponds exactly to the height of a column of spots but also that all the spots be substantially identical in form and in intensity.
In the axis corresponding to the height of the column of optical gates, the illumination of the modulator should preferably be uniform and the impinging light beam must show very little divergence so that substantially all the light crossing the modulator enters the pupil of the projection optics. The other axis, width axis, demands a highly collimated beam to produce a column of spots measuring 10 microns or less in width. This necessitates the use of a laser source having a resolution of the order of one micron in this width axis.
To expose highly sensitive media such as silver halides surfaces, electrophotographic plates or films, it is possible to use low-power laser diodes. In this case, the gaussian intensity profile can be transformed by optical means as described in U.S. Pat. No. 4,746,942.
Temperature sensitive polymers (offset plates or any heat sensitive media) demand exposure to stronger luminous power. This can be obtained by the use of high power laser diode(s) to illuminate the spatial modulator. The emission profile location of the diode in the direction of the height (also named "fast axis") is of the order of a micron and has a gaussian emission profile. This must match the width of the column of spots.
In the other direction, particularly high power diodes are available with different types of emitters and configurations. With one-emitter diodes the width of the emitted beams can measure from one to several hundred microns. The most powerful diodes have a multiplicity of emitters located along a one-centimeter bar. They can be located serially or in regularly spaced groups. The angular divergence in this axis is small but the angular emission profiles are irregular and vary from a diode to another. Thus, the distribution of the radiant energy is not uniform.
U.S. Pat. No. 5,517,359 is directed to a complex apparatus for coupling a broad emitting area laser diode made of multiple emitters operating in parallel to a linear light valve. The apparatus images each one of the emitters onto the linear light valve, superimposing them in order to increase the immunity to defects occurring in any individual emitter.